1. Field of the Invention
The present invention relates to a high-dispersion sol or gel of monoclinic zirconia supermicrocrystals having a diameter smaller than 100 .ANG. which has never been known before, and a method of producing the same. More particularly, it relates to a high-dispersion sol or gel of monoclinic zirconia supermicrocrystals, said sol or gel containing monoclinic zirconia supermicrocrystals of extremely fine and uniform rodlike shape or ellipsoidal shape smaller than 100 .ANG. in diameter or width, in a concentration higher than 0.1 mol/liter and a method of producing the same. The term "zirconia" used herein denotes what is commonly meant by zirconia which consists of ZrO.sub.2 or contains ZrO.sub.2 with a small amount of other metal oxides such as HfO.sub.2 in the form of solid solution.
2. Description of the Prior Art:
Sol dispersions of monoclinic zirconia microcrystals have been available in various forms; however, they are mostly sols of agglomerate secondary particles. Even in the case of sol composed of discrete particles, the size of the particle is greater than 100 .ANG.. There has not yet been produced high dispersion sol of supermicrocrystals smaller than 100 .ANG..
There is a known process of producing a hydrosol of zirconia by heating an aqueous solution of a zirconium salt at 120.degree. to 300.degree. C. under a hydrothermal condition (U.S. Pat. No. 2,984,628). This process is a very basic one. According to this U.S. Patent, it is claimed that the concentration of the sol should preferably be 0.1 to 2.0 mol/liter, the reaction is substantially completed within 1 to 4 hours, and a maximum concentration of 4 mol/liter is possible and a maximum reaction time of 10 hours is permissible. This U.S. patent, however, discloses nothing about the crystallizability, shape, and dispersibility of the resulting particles. In fact it describes nothing about the process of producing high dispersion sol or translucent gel thereof composed of supermicrocrystals smaller than 100 .ANG. as disclosed in the present invention.
Being aware of the characteristic properties and usefulness of high dispersion sol of zirconia supermicrocrystals smaller than 100 .ANG., the present inventor carried out a series of research analyses, which led to the finding that there is an important region outside the scope of the above-mentioned U.S. Patent The present invention was completed on the basis of this finding.